Other Comprehensive Income (Loss) (Component of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, before tax amount	$ 277,926	$ (2,143,567)	$ (231,355)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, before tax amount	(649)	(737)	(971)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, before tax amount	(66)	0	(69)
Total securities available for sale and transferred securities before tax amount	277,211	(2,144,304)	(232,395)
Change in the net actuarial gain/loss before tax amount	9,278	(5,005)	16,593
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	3,479	2,964	6,116
Total defined-benefit post-retirement benefit plans	12,757	(2,041)	22,709
Other comprehensive income (loss), before tax	289,968	(2,146,345)	(209,686)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, tax expense (benefit)	58,364	(450,149)	(48,585)
Securities available for sale and transferred securities, change in net unrealized gain on securities transfers to held-to-maturity, tax expense benefit	(136)	(155)	(204)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, tax expense (benefit)	(14)		(14)
Total securities available for sale and transferred securities, tax expense (benefit)	58,214	(450,304)	(48,803)
Change in the net actuarial gain/loss, tax expense (benefit)	1,948	(1,051)	3,485
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit) , tax expense (benefit)	731	622	1,284
Total defined-benefit post-retirement benefit plans, tax expense (benefit)	2,679	(429)	4,769
Total other comprehensive income, tax expense (benefit)	60,893	(450,733)	(44,034)
Securities available for sale and transferred securities, change in net unrealized gain/loss during the period, net of tax amount	219,562	(1,693,418)	(182,770)
Securities available for sale and transferred securities. change in net unrealized gain on securities transfers to held-to-maturity, net of tax amount	(513)	(582)	(767)
Securities available for sale and securities transferred, reclassification adjustment for net (gains) losses included in net income, net of tax amount	(52)		(55)
Total securities available for sale and transferred securities, net of tax amount	218,997	(1,694,000)	(183,592)
Change in the net actuarial gain/loss, net of tax amount	7,330	(3,954)	13,108
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit), net of tax amount	2,748	2,342	4,832
Total defined-benefit post-retirement benefit plans, net of tax amount	10,078	(1,612)	17,940
Other comprehensive income (loss), net of tax	$ 229,075	$ (1,695,612)	$ (165,652)